Environmental exposure to polychlorinated biphenyls (PCBs) and other organochlorine compounds occurs worldwide, but considerable uncertainty exists regarding adverse human health effects caused by long-term exposures. Recent evidence suggests that certain PCB congeners, was well as some other organochlorine compounds, may play a role in the pathogenesis of beast cancer in women. This effect could be mediated if these pollutants act as xenoestrogens. Thr proposed project therefore intends to determine whether breast cancer patients had an increased body burden of organochlorine compounds, as reflected in the serum concentration before the disease developed. The study will utilize the unique opportunities created by the long-term population studies carried out i Denmark. The women belong to three major cohorts examined in 1976 and 1982, and a case group of women who later d enveloped breast cancer will be matched with a control group (two controls for each case). A total of 215 cases are expected. Extensive baseline data on relevant risk factors as well as frozen serum samples are available from the original e examinations, and follow-up through the Danish Cancer Registry and the mortality data base can be extended through 31 December 1992 or later. Organochlorine analyses will be carried out at Centers for Disease Control in Atlanta. Information on major confounders will be included in the statistical analyses where adjusted odds ratios will be calculated. The general homogeneity of the Danish population, the detailed assessment of potential confounders prior to disease development, and the general completeness and validity of the data, will ensure that this project will provide crucial information for risk assessment of environmental PCB and other organochlorine exposure.